The name Eutheria comes from the Greek words eu- "well-developed" and ther "beast". When Eutheria was introduced by Thomas Henry Huxley in 1880, he meant for it to be more broad in definition than its precursor Placentalia. Some use Eutheria as a total group which includes the crown group Placentalia and extinct mammals which are closer to Placentalia than to Marsupialia.

Nevertheless, all Eutherians are placental mammals. This means that a Eutherian fetus is nourished during gestation by a placenta. Eutherians are also viviparous, meaning that the offspring are carried in the mother's womb until fully developed.

Because of this, Eutherians are different from other mammal groups such as monotremes and marsupials which are not placental. Monotremes, for instance, lay eggs which protect developing young until they are fully developed. Marsupials give birth to partially-developed young who then migrate to a special pouch in the mother's body in which the young continue their development. (Some exceptions do exist. Bandicoots for instance, which are marsupials, develop small placenta-like structures during gestation.)

The earliest known eutherian species is the extinct Eomaia scansoria from the Lower Cretaceous of China. It is undoubtedly a member of Eutheria, but the hips of the animal were too narrowly built to have allowed the birth of well-developed young. Thus it is unlikely that a placenta greatly contributed to the development of Eomaia scansoria's young before they were born.